In the proposed project our objectives are: 1. To examine the oncogenic potential of JC papovavirus which has been isolated from human brains with progressive multifocal leukoencephalopathy and also from the urine of immunologically impaired or immunosuppressed patients. After inoculation by various routes, gross autopsies and light-microscopic studies will be performed on Syrian hamsters, which have been found previously to be highly susceptible to JC virus tumor induction. Selected neoplasms will be studied by electron microscopy. Sequential details of JC virus induced medulloblastomas will be studied by light and electron microscopy and by immunofluorescent technique. Following a positive pilot study, JC viral oncogenicity will be examined more extensively in a series of owl monkeys, with emphasis on the central nervous system. 2. To conduct a first test concerning possible teratogenic effects of JC virus in the Syrian hamster. 3. To extend our search in current human brain autopsy material for diseases of slow viral nature.